This invention relates generally to switched mode power supplies and is particularly directed to an arrangement for automatically correcting for variations in the operation of a controller circuit in a switched mode power supply.
A switched mode power supply generally includes at least the following components: a switch, an inductor, a rectifier, a capacitor and a load. During that portion of the period in which the switch conducts, a current originating from an input voltage source passes through the inductor so that energy which is derived from this source is stored in the inductor. During the other part of the period, in which the switch is nonconducting, the energy stored in the inductor produces a current through the rectifier, which current recharges the capacitor and also replenishes the energy losses arising from the load. By adjusting the conducting period of the switch relative to the length of the cycle, the output DC voltage across the load can be independent of various power supply inputs.
Various control circuits have been devised for regulating the operation of the conducting period of the switch which typically is a transistor. These control circuits monitor and control various operating parameters of the power supply such as the rectified AC line voltage provided thereto, the duty cycle of the switching transistor, the DC output voltage, etc. For various reasons including reduced space, reduced power consumption, increased reliability, and a reduction in heat generated by the power supply, these control circuits are increasingly of the integrated type. One example of such an integrated control circuit for use in a switched mode power supply is disclosed in U.S. Pat. No. 4,228,493. Various operating parameters are provided to these integrated control circuits which respond by providing various control outputs for regulating power supply operation.
Switched mode power supply integrated circuit controllers, however, generally exhibit variations in their own operating characteristics. Variations between individual controller chips of the same type arise from production run variations in their manufacture. For example, these controllers generally output a first reference voltage which is compared with a second voltage representing the power supply's output for providing a correction signal back to the controller in regulating power supply operation. This reference voltage may vary from time to time or may be different for different controllers. In addition, variations in the operation of the external circuitry to which the reference voltage is provided may give rise to additional variation in controller operation and power supply outputs. The prior art has addressed these variations and instabilities in power supply controller operation by providing a manual adjustment, typically in the form of a potentiometer, in attempting to achieve predictable, stable operation of the power supply under various operating conditions. One example of such a manual adjustment by means of a potentiometer can be found in the Dec. 21, 1978 issue of Electronics in an article entitled "Flyback Converters: Solid-state Solution to Low-Cost Switching Power Supplies." However, this manual adjustment is time consuming, introduces the possibility of human error, and, because it requires human intervention, increases the manufacturing cost of the power supply.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a self-adjusting switched mode power supply which automatically compensates for variations in power supply controller operation. The correction circuit of the present invention provides a highly accurate error signal to the controller which is independent of controller circuit operation and closely tracks variations in power supply output.